The present invention generally relates to thermal printing apparatuses, and more particularly to a thermal printing apparatus which prints with a desired gradation by controlling application times of a constant current applied to heating elements of a thermal printing head.
Generally, a thermal printing apparatus employs a polyester ink film which has a thickness of approximately five to six microns and has one surface thereof coated with a kind of ink which melts when heated. The ink film is provided so that a first surface thereof coated with the ink opposes a recording sheet, and a thermal printing head comprising heating elements is arranged so as to oppose a second surface of the ink film not coated with the ink. When the thermal printing head makes contact with the second surface of the ink film and a current is applied to the heating elements of the thermal printing head, those heating elements applied with the current are heated, and the ink coated on the ink film is melted at positions corresponding to the heating elements which are heated so as to transfer the melted ink onto the recording sheet. Usually, the thermal printing head comprises a plurality of heating elements which are arranged in-line, and the current is successively applied to the heating elements depending on printing data. An image such as characters, figures, picture and the like is printed on the recording sheet by the heating elements which print dots on the recording sheet.
The gradation of the image printed on the recording sheet is determined by the areas of the dots which are printed by the heating elements. The area of each dot is determined by the magnitude of the current applied to each heating element, or by an application time of a constant current applied to each heating element. The heat generated by the heating element increases as the magnitude of the current applied thereto increases or as the application time of the constant current applied thereto increases, and the area of the printed dot accordingly increases. Hence, it is possible to control the gradation of the printing by controlling the application time of the constant current applied to each heating element, that is, by controlling the the heating time of each heating element.
In a conventional thermal printing apparatus which prints one picture element of an image by use of one heating element, when the relationship between the application time of the current applied to the heating element and the density of the dot printed by the heating element is plotted to obtain a graph, it can be seen from the graph that a time period in which the melting quantity of the ink greatly changes is extremely short and the density of the dot rapidly changes within this extremely short time period. For this reason, in order to carry out the printing with a large number of gradation levels, it is necessary to assign a large number of gradation levels within the extremely short time period, and it is extremely difficult to carry out the printing with a large number of gradation levels.
Accordingly, it is possible to conceive a thermal printing apparatus which prints one picture element of the image by use of two heating elements, for example. In this case, compared to the case where one picture element is printed by use of one heating element, it is possible to carry out the printing with a larger number of gradation levels. However, when the current is applied to a first one of the two heating elements and the current is thereafter applied in succession to a second one of the two heating elements, the rate with which the density of the printed picture element changes does not become constant with respect to the application time of the current. Therefore, as will be described later on in the present specification, there is a problem in that it is difficult to control the gradation of the printing.